First the Giants flushed the Patriots — then Big Blue’s fans did a whole lot of flushing of their own.

In the moments after New England’s dreams of Super Bowl glory went down the drain with Tom Brady’s failed last-second Hail Mary toss, toilet use spiked a staggering 13 percent in the city, according to Department of Environmental Protection.

It was sweet relief for millions of fans — in more ways than one.

The toilet-bowl shuffle — which came after tense hours of holding it in — was so strong that the 30-foot-deep water level in the Hillview Reservoir in Yonkers actually decreased by 2 inches after the contest.

“This was certainly a significant jump — especially for around 10 p.m. on Sunday night. It’s only going to happen on the Super Bowl,” said a stunned Jim Roberts, deputy commissioner for the city’s DEP.

Sunday’s gametime water usage was nowhere close to a week earlier, when the Giants were off and the only football on television was the little-watched Pro Bowl.

The amount of water flushed by city residents before Super Bowl XLVI started Sunday looks almost identical to the prior week, DEP records show.

But as the game kicked off at 6:30 p.m., Giants fans started gluing their faces to TV screens in their homes and in bars around the city — and the amount of water use started dropping dramatically.

And when the action picked up in the first quarter, water use was down more than 7 percent over the previous week, and it would only go down from there.

“When the game starts, no one is leaving their seats unless they’re getting something to eat or a soft drink or a beer,” Roberts said.

By the time the second quarter ended, water use was down more than 8 percent from the non-football Sunday before.

Even though Big Blue was trailing at the half, at least fans found some relief in their own super bowls.

That’s when water use jumped nearly 4 percent, before dropping back down about the same amount as Madonna took the stage.

It then spiked some 4 percent just before the second half began.

The water use continued to drop as the game grew more intense — reaching its low point during Mario Manningham’s sideline catch.

At that point, water use was 9.6 percent lower than the week before.

After the G-men took the lead late in the fourth quarter, New Yorkers must have thought the game was done — water use immediately started rising.

By 10:20 p.m., as the team was accepting its Lombardi Trophy, water use was 5.4 percent higher than a week before.

Roberts said the stats have verified what engineers have seen for years. “We kind of understand what is happening,” he said.